Commonly employed fabrication techniques for displays and polymer based devices or other semiconductor electronic devices involve several imaging steps. A substrate coated with a resist or other sensitive material is exposed to radiation through a photo-tool mask to effect some change. By nature these fabrication processes involve a large number of separate steps, each step commonly having a finite risk of failure, thus reducing the overall process yield and increasing the cost of fabricating the finished article. A specific example is the fabrication of color filters for flat panel displays. Color filter fabrication can be a very expensive process because of the high cost of materials and low process yield. Traditional photolithographic processing involves applying color resist materials to a substrate using various coating techniques such as spin coating, slit and spin and spin-less coating. The material is then exposed via a photo-tool mask followed by a development process.
Direct imaging has been proposed for use in the fabrication of displays and in particular color filters. U.S. Pat. No. 4,965,242 to DeBoer et al. describes a dye transfer process for making a color filter element. A dye receiving element is overlaid with a dye donor element which is then imagewise heated to selectively transfer the dye from the donor to the receiver. The preferred method of imagewise heating is by means of a laser beam. Diode lasers are particularly preferred for their ease of modulation, low cost and small size.
Direct imaging has the potential for replacing a multiplicity of steps associated with traditional photolithographic processes with a single imaging step. A downside of direct imaging is that the laser beam is required to scan over the entire surface of the substrate. This necessitates very fast imagewise scanning of the substrate in order to preserve the advantages of direct imaging over flood exposure. Flood exposure through a photo-tool mask is by nature a very fast imaging process because a small substrate may be exposed at once, or a series of quick step repeat exposures may be used for larger substrates. One way to increase the speed of the direct imaging is to expose the substrate simultaneously with a plurality of laser beams. U.S. Pat. No. 6,146,792 to Blanchet-Fincher et al. describes the production of a durable image on a receiver element, such as a color filter. The laser head suggested in the examples consists of thirty-two 830 nm laser diodes, each having approximately 90 mW of single-mode output.
Imaging heads with even more channels are now commonly available, exemplified by the SQUAREspot® thermal imaging heads manufactured by Creo Inc. of Burnaby, British Columbia, Canada. These imaging heads are available with up to 240 independent imaging channels each channel having upwards of 100 mW of optical output power per channel. Such imaging heads offer imaging of a small 370×470 mm color filter substrate in around 3 minutes for a media sensitivity of 450 mJ/cm2.
Further improvement in imaging speed is often frustrated by the trade-off between imaging resolution and speed. Color element edge definition requirements dictate that a small pixel size be used (i.e. high resolution imaging). However, the smaller the pixel, the longer it takes to scan over the substrate to effect the imagewise exposure. The availability of imaging heads with progressively more channels does not entirely address this problem since the required number of channels are difficult to provide in an economical and practical imaging system.
The speed/resolution trade-off, coupled with the industry trends towards processing larger and larger substrates presents a unique challenge for direct imaging systems. Larger substrates are not only of application in producing larger displays but also in improving the economy and yield of smaller display panel fabrication. A large substrate may be processed and later separated into a number of smaller panels. Having more panels per processed substrate reduces the chance that an entire substrate that has been processed will be un-unusable (2 faults on a 4 panel substrate is a 50% yield while the same 2 faults on a 12 panel substrate is an 83% yield). In the display fabrication industry, so-called “sixth generation” flat panel display substrate sizes are around 1500×1800 mm. For the example above having 450 mJ/cm2 media sensitivity the imaging time with the 240 channel imaging head would be in the region of 45 minutes, which is prohibitively long, particularly when compared to flood exposure, which is only marginally slower for a larger substrate where relatively large areas are imaged in a series of step and repeat exposures.
There remains a need for higher productivity direct imaging techniques used in the fabrication of color filters and other polymer based electronic devices.